1. Field of the Invention
The present invention relates to a frequency offset estimating apparatus and method applicable for a digital television receiver and, more particularly, to a frequency synchronization apparatus and method in an orthogonal frequency division multiplexing (OFDM) system for estimating a frequency offset by using a correlation value according to an echo channel, a delay path with effective power, to reduce inter-symbol interference (ISI), to thereby improve degradation of estimation performance of the frequency offset.
2. Description of the Related Art
In general, an OFDM system is disadvantageous in that it is sensitive to a frequency offset generated due to mismatching of a Doppler shift or a transmitter/receiver oscillator in a radio channel.
The frequency offset ruins orthogonality of subcarriers with respect to a reception signal to cause an inter-carrier interference (ICI) phenomenon, and affects the amplitude as well as a phase rotation of signals. In addition, Distortion of the amplitude brings about an effect that the signal is scattered like noise even in an environment without noise, aggravating the signal distortion.
Therefore, in order to maintain a stable system performance in the OFDM system, frequency synchronization should be precisely performed, and in order to precisely perform frequency synchronization, a frequency offset should be precisely estimated.
FIG. 1 is a view illustrating the related art frequency offset estimating method.
With reference to FIG. 1, the related art frequency offset estimating method uses the fact that a guard interval 11 and a rear portion 13 of an effective symbol interval 12 are alike (equivalent) in an OFDM symbol 10 of a time domain, based on which a frequency offset is estimated through a phase variation in the two intervals.
A frequency offset estimated according to the related art frequency offset estimating method may be represented by Equation 1 shown below:
                              ɛ          ^                =                              1                          2              ⁢              π                                ⁢          arc          ⁢                                          ⁢          tan          ⁢                      {                                                            y                  *                                ⁡                                  [                                      n                    ⁢                                          -                                        ⁢                                          N                      GI                                                        ]                                            ⁢                              y                ⁡                                  [                                      n                    +                                          N                      ⁢                                              -                                            ⁢                                              N                        GI                                                                              ]                                                      }                                              [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, ‘N’ is the size of a fast Fourier transform (FFT), NGI is the size of the guard interval, * is a conjugate complex number, ‘k’ is the number of accumulated symbols, and {circumflex over (ε)} is the estimated frequency offset.
FIG. 2 is a graph of a delay profile obtained by simply modeling an echo channel.
With reference to FIG. 2, a root mean square error (RMSE) performance in the absence of echo in a considered channel environment is shown by G1 (Power=1:0) and G2 (Power=0.5:0), and an RMSE performance in the presence of echo is shown by G3 (Power=0.5:0.5).
However, the related art frequency offset estimating method has a problem in that the performance is drastically degraded if echo exists compared with the case without echo.